A structure for further increasing the electron mobility and the electron density of a high electron mobility transistor (HEMT) includes a pseudomorphic high electron mobility transistor (P-HEMT) structure. With the most made of their high mobility characteristic, P-HEMTs having a Schottky gate structure or a pn junction gate structure are widely used in high-frequency communication devices.
Patent Document 1 and Patent Document 2 disclose epitaxial wafers for P-HEMTs. The epitaxial wafers disclosed in these documents use an InGaAs layer as a strained channel layer, and AlGaAs layers as a front-side and a back-side electron supply layers. Patent Document 1 describes that the In composition of the strained channel layer is 0.25 or more. Patent Document 1 further describes that optimization of the In composition and the thickness of the strained channel layer would result in an electron mobility of the strained channel layer of 8300 cm2/V·s or higher at 300 K (the maximum value that is explicitly shown is 8990 cm2/V·s). Patent Document 2 describes that optimization of the In composition and the thickness of the strained channel layer would result in an emission peak wavelength of the strained channel layer of 1030 nm or more at 77 K (the maximum value that is explicitly shown is 1075 nm). The electron mobility was measured by Hall measurement (Van der Pauw method). Patent Document 3 discloses an interfacial structure between an insulator and a compound semiconductor. Patent Document 3 discloses that this interfacial structure between an insulator and a compound semiconductor includes a compound semiconductor, a spacer layer provided directly or indirectly on the surface of the compound semiconductor, and an insulating layer provided directly or indirectly on the spacer layer, and that the spacer layer is made of a semiconductor substance having a bandgap wider than the badgap of the compound semiconductor.    Patent Document 1 Japanese Patent Application Publication No. 2004-207471    Patent Document 2 Japanese Patent Application Publication No. 2004-207473    Patent Document 3 Japanese Patent Application Publication No. H10-275806